Chlorination and oxidation of heparin and hyaluronan by hypochlorous acid and hypochlorite anions: effect of sulfate groups on reaction pathways and kinetics

Hypochlorous acid (HOCl), produced in inflammatory conditions by the enzyme myeloperoxidase, and its anion hypochlorite (OCl⁻) exist in vivo at almost equal concentrations. Their reactions with hyaluronan and heparin (as a model for sulfated glycosaminoglycans in the extracellular matrix) have been studied as a function of pH. The major product in these reactions is the chloramide derivative of the glycosaminoglycans. Spectral, chloramide yield, and kinetic measurements show sharply contrasting behavior of heparin and hyaluronan and the data allow the calculation of second-order rate constants for the reactions of both HOCl and OCl⁻ for all reaction pathways leading to the formation of chloramides and also oxidation products. By comparison with hyaluronan, it can be demonstrated that both N-sulfate and O-sulfate groups in heparin influence the proportions of these pathways in this glycosaminoglycan. Evidence is also given for further oxidation pathways involving a reaction of HOCl with the chloramide product of hyaluronan but not with heparin. The significance of these results for the mechanisms of inflammation, particularly for fragmentation of extracellular matrix glycosaminoglycans, is discussed.